Field of the Invention
The present invention relates to a liquid discharge head and a liquid discharge method, and more particularly relates to a liquid discharge head liquid circulates before and after discharge orifices.
Description of the Related Art
In liquid discharge heads that discharge liquid such as ink or the like, the liquid may become concentrated and thicken near discharge orifices, due to volatile component in the liquid being discharged from the discharge orifices evaporating. This can change the discharge speed of droplets, and droplet landing accuracy may become poorer. Thickening of the liquid is particularly marked in cases where an intermission period from having discharged a droplet until discharging the next droplet is long, or in cases where the content of solids in the liquid is high. In a worst-case scenario, defective discharge may occur due to the increased flow resistance of the concentrated liquid.
Circulating liquid supplied to the liquid discharge head over a circulation path is known as one measure to deal with this liquid thickening phenomenon. Liquid discharge heads that have recording elements generating thermal energy are disclosed in Japanese Patent Laid-Open No. 2001-205814, and “Carolyn Ellinger and Yonglin Xie in ‘Captive Continuous Inkjet’, September 2013, 29th International Conference on Digital Printing Technologies” (hereinafter “ELLINGER”), which is non-patent literature (hereinafter, this system for liquid discharge heads may be referred to as “thermal system”). A liquid is circulated through liquid channels formed between a discharge orifice forming member where discharge orifices are formed, and a substrate where the recording elements are formed, to prevent the discharge orifices from becoming clogged from evaporating liquid. Japanese Patent Laid-Open No. 2001-205814 describes the ink being circulated at a flow velocity of 50 to 2000 μm/s, thereby discharging bubbles residing near the heat-generating elements to a downstream region. ELLINGER describes circulating ink at a faster flow velocity.
The Present Inventors have found through studies that regarding the configuration described in ELLINGER relating to continuous inkjet technology, the high speed of the circulation flow velocity affects bubbles generated by driving the recording elements. Specifically, the bubbles may not be formed symmetrically regarding the center of the discharge orifice, and the discharge direction of the droplet may incline as to a direction perpendicular the face of the discharge orifice forming member where the discharge orifices are formed (hereinafter “discharge orifice forming face”). Particularly, the height of a channels communicating with the pressure chambers in the thermal system, where bubbles are generated and droplets are discharged, is low in comparison with piezoelectric systems, and the discharge orifices are arrayed in high density, so the flow resistance is great. Accordingly, the flow resistance before and after the discharge orifices is great, and bubbling readily occurs asymmetrically. Asymmetric bubbling easily causes the discharge direction of the droplet to be inclined as to the direction perpendicular to the discharge orifice forming face.
On the other hand, Japanese Patent Laid-Open No. 2001-205814 describes the liquid being circulated at a flow velocity of 50 to 2000 μm/s, but the flow velocity is slow, so even though residual bubbles can be moved downstream, suppressing thickening of liquid due to evaporation of liquid from the discharge orifices is difficult. Thickened liquid near the discharge orifices can change the discharge speed of droplets, and the landing positions of the droplets may deviate from the intended landing positions. This problem becomes particularly conspicuous in cases where the temperature of the liquid discharge head is high and the rate of evaporation is fast, and in cases where the concentration of solids in the liquid is high.